1. Field of the Invention
The present invention relates to a method for two-level variable chip rate spreading and despreading used in CDMA (Code Division Multiple Access) system.
2. Description of the Prior Art
In CDMA system, many users share the same frequency band and time slot. Multiple access transmission can be implemented through the allocated spread sequence, i.e. the address code. In this case, the interference acting on user receivers in the system generally comprises channel noise, self-interference (SI) caused by a user signal multipath transmission, multiple access interference (MAI) caused by the other users in a cell, and adjacent cell interference (ACI) caused by users in adjacent cells. In ideal case, spreading code sets used in CDMA system should have the following correlation properties: the autocorrelation function of each spreading sequence code is an impulse function, that is, the value should be zero at passim except the zero time delay. The value of the cross correlation function for each pair of spreading sequence codes should be zeros passim. In this case, there is only channel noise in the system. Although it has been proved that the spreading sequence code sets having such ideal correlation properties do not exist, however, except the first kind of channel noise, the rest three kinds of interference can be effectively suppressed by selecting a appropriate address code and a spreading method, thereby a better system performance can be achieved.
In terms of the existing CDMA systems, a two-layer spreading method is generally employed. In a downlink, the first layer spreading uses an orthogonal code such as Walsh sequence (e.g. IS-95, TD-SCDMA, CDMA2000, etc) or OVSF (orthogonal variable spreading factor) sequence (e.g. WCDMA) as the spreading address codes (i.e., channel code) so as to distinguish channels in a cell and suppress interference from other users within the cell. The second layer spreading uses generally PN (pseudo-random) sequence such as m sequence (for example, IS-95, CDMA 2000) and Gold sequence (for example, WCDMA) as an address code (i.e., scrambling code) so as to distinguish the channels of different cells. Similarly, in an uplink, such a two-layer spreading method comprising of orthogonal sequence and pseudo-random sequence (WCDMA, TD-SCDMA, etc) can also be used. However, the orthogonal sequence sets can be used only in different channels of the same user and the pseudo-random sequences for each of users within the same cell should be different. The characteristics of the conventional two-layer spreading method are in that the chip rate has already reached the chip rate required by the system after it has been spread with the first layer orthogonal channel code. The second layer spreading is just an XOR operation of the chips come through the first layer spreading and scrambling chips with the same rate, the chip rate is not changed and the spreading is not executed. In other words, the scrambling code of the second layer is only used as an address code and it plays no role in spreading. Such a two-layer method has advantages of which the match of the channel code and the scrambling code increases the number of available channels in the system, MAI (from other users in downlinks and from other channels of the same user in uplinks) on same path can be removed completely with the orthogonal properties of the orthogonal code, PN sequence can randomize the interference from other paths and other cells (in the case of uplinks, it is users), and give rise to greater system gain.
It can be seen from above description, the purpose for the use of orthogonal sequence in CDMA system is to maintain the orthogonalilty of the user signals so as to suppress interference. However, in practice, the improvement of performance produced by the orthogonal sequence is limited because the synchronization may be destroyed by the influence of the transmission or channels. In terms of downlinks, the useful signals which are received by the mobile station receiver are naturally synchronized with the interference from the users of the same cell, therefore, such a destruction usually results from multipath propagation. Especially, when the number of fingers of a RAKE receiver is smaller than that of multiple paths of a propagation channel, the system performance degradation due to such destruction will be worse. In reverse links, even if a special synchronous signal (or channel) is sent, because of the different distances between mobile stations and the base stations, the time that the signals arrived at the base stations is different. Based upon the reasons mentioned above, in the reverse links of most systems, different users use different PN sequences, while the same PN sequence and different orthogonal sequence are employed between the different channels of the same user. Similar to the case of downlinks, although the different channel signals of the same user are naturally synchronized, their orthogonality is still subject to the influence of multipath propagation.
In order to solve the problem which the orthogonality of orthogonal sequence is destroyed by synchronous error, Chinese Patent Application No. 00103282.8 proposes a sequence which has a certain width for zero correlation zone (ZCZ), hereinafter refer to it as ZCZ sequence, in the vicinity of zero time delay. ZCZ is used as an address code in CDMA system. Interference can be effectively suppressed as long as the time delay offset between the useful signals and the interference signals is limited within the zero correlation zone Z because the autocorrelation function and cross correlation function of ZCZ sequence remains zero in a certain area. At this time, interference can not be completely removed because the magnitude of the interference depends on the partial correlation property of the sequence rather than on the periodic correlation property of the sequence. Therefore, in Chinese Patent Application No. 00103282.8, an additional guard chip is added into the spread signal so as to ensure the cycle zero correlation property to be available. Through simulation and analysis, it indicates that a good system performance can be still achieved even if guard chip is not added because the partial correlation property of ZCZ sequence in zero correlation zone is also excellent. Thus, for downlinks, all the multipath components which time delays are smaller than Z can be effectively suppressed. In uplinks, when the radius of a cell is small (for example, microcell system), as the non-synchrony caused by the distance can be controlled within several chips, the system which meets such a requirement of synchrony is referred to as quasi-synchronous system. For a quasi-synchronous system, as long as the delay does not exceed the zero correlation zone Z of ZCZ, the interference among the users using different ZCZ sequences can still be effectively suppressed. Therefore, interference in the system can be effectively suppressed by using ZCZ sequence as spreading address code, and a good system error code performance can be achieved.
According to above analysis, system performance can be improved by using ZCZ sequence as an address code in a quasi-synchronous system. However, because the second layer spread spectrum uses pseudo-random sequence as the interference code, if the channel code in the current CDMA system is replaced by ZCZ sequence in order to provide channel division between cells, the action of the zero correlation zone of ZCZ can not be shown when signals between the users are non-synchronous due to multipath or transmission. In this case, only the zero correlation property at the zero time delay can be used. At that time, ZCZ sequence functions just as a general orthogonal sequence, and the full potential of zero correlation zone of ZCZ sequence is not exerted.